1. Field of the Invention
The invention relates to an exhaust gas control system for an internal combustion engine and a method for recovering a filter thereof.
2. Description of the Related Art
Exhaust gas discharged from an internal combustion engine contains particulate matter that mainly includes carbon. A technology is known, in which a particulate filter (hereinafter, referred to as “filter”) for capturing the particulate matter is provided in an exhaust system in an internal combustion engine in order to prevent the particulate matter from being discharged to the atmosphere.
In such a filter, when the amount of captured particulate matter increases, the pressure of the exhaust gas is increased due to clogging of the filter, which leads to a decrease in engine performance. Therefore, the exhaust gas temperature on the upstream side of the filter is increased, and the captured particulate matter is oxidized and removed, whereby the exhaust gas control performance of the filter is recovered (hereinafter, this process will be referred to as “filter forced recovery”).
It is known that a filter forced recovery region and a filter continuous recovery region are included in an entire range of an engine operating state decided by a rotational speed and torque of the internal combustion engine. The filter forced recovery region is an operating state range where the exhaust gas temperature is relatively low, and the amount of the particulate matter deposited in the filter increases if the filter forced recovery is not performed. The filter continuous recovery region is an operating state range where the exhaust gas temperature is relatively high, and the amount of the particulate matter deposited in the filter decreases even if the filter forced recovery is not performed.
Japanese Patent Laid-Open Publication No. 2003-155919 discloses a technology in which the amount of captured particulate matter in the filter is accumulated in the case where the operating state of the internal combustion engine belongs to the filter forced recovery region. In the case where the operating state of the internal combustion engine belongs to the filter continuous recovery region, the accumulation of the amount of the captured particulate matter is stopped, and the filter forced recovery is performed when the accumulated amount of the captured particulate matter becomes equal to or larger than a predetermined amount.
In the case where the operating state of the internal combustion engine belongs to the filter continuous recovery region, the amount of the particulate matter deposited in the filter tends to decrease even if the filter forced recovery is not performed. However, when it takes long time to oxidize the particulate matter, the particulate matter remains in the filter for a long time without being oxidized. Therefore, a relatively large agglomeration of ash may be formed after the particulate matter is oxidized, and the agglomerated ash may be deposited without being removed from the filter. Also, oxidation of the particulate matter may become insufficient.